An electric field electron emission element is a cold-cathode which consumes less energy and has an extended life longer than a thermionic emission element which has to be heated. The electric field electron emission element needs to employ an electron emission material which has such a small curvature at a vertical hem thereof that an electron can be emitted at a low voltage at an improved electron emission efficiency.
In this regard, recently, carbon fiber materials such as carbon microfiber have been noticed as an electron emission material of the electric field electron emission element. Out of these materials, a carbon nanofiber such as carbon nanotube of a nanoscale size is particularly distinguished. A carbon nanofiber, whose external diameter ranges approximately from 1 to 100 nm and length of a few μm's, can be properly shaped to perform the field electron emission at a low voltage. Further, since the component material, i.e., carbon, is chemically stable and mechanically strong, it is an ideal material for electron emission element.
Conventionally, a carbon fiber has been produced by employing a laser ablation method, an arc discharge method using arc discharge between graphite electrodes in a vacuum state or in inert gas, a CVD (chemical vapor deposition) method, or the like. Since the CVD method is capable of producing a carbon fiber with a better regularity than other methods, it has received more attention recently.
In case a carbon fiber, e.g., a carbon nanotube, is used as an electron emission material, the carbon nanotube is desirably arranged in a direction along an electric field (generally, a direction perpendicular to a substrate on which the carbon nanotube is arranged) for concentration of the electric field. Since the carbon nanotube has a thread shape, however, most carbon nanotube's vertical hems are not arranged perpendicular to the substrate when the carbon nanotubes are simply deposited on the substrate; and, therefore, irregularity occurs when a low voltage is applied.